Thanks for all of the replies; you all have given me a lot of ideas.
First off, my name is Ted Bruce, and my amateur radio callsign is KX4OM. I hold an Advanced class license, and I've been a ham since 1960. I also gained the FCC 1st Class Commercial Radiotelephone Operator license with Radar Endorsement in 1979.
The antenna I'm designing is for hobby use and non-commercial. The design is to be freely shared with the ham radio community. It is an 8-foot long portable vertical antenna that breaks down into 2-foot long sections for transit. Removable coils wound on CPVC and attached with 1/4 x 20 hardware to the coupler nuts inside the tube allow operation on several bands. It is not designed for permanent installation.
The idea of using brass pipe nipples is one adopted by Phil Salas, AD5X, a EE and communications and antenna designer. Also, James Bennett, KA5DVS designed an antenna (of which a version is now a commercial product) that used two 1-foot long 1/4-inch diameter aluminum rods, tapped with 1/4 x 20 thread at the ends and joined by the same coupler nuts I'm using. His designed used a thin 6-foot Radio Shack whip as the extension of the antenna above the removable coils to an overall length of 8 feet. When I built that antenna, I found that tapping the rods was not only difficult to achieve and created weak points at the point that they were joined by the coupler nuts. I used mine during a contest back in May in horrible weather, and it blew over and bent the cheap Radio Shack whip and the aluminum rod sections as well.
In coming up with a hopefully better solution, I recalled from my Navy Nuclear Power program training that for the same material and mass, a tube structure is stronger than a rod. The 2-foot long solid rods I used initially per the published design are insubstantial. I can bend them and wrap them around my hand, in fact.
Another ham down here in Georgia and I have been corresponding lately on this topic. His antenna is 5/8-inch aluminum with stainless steel couplers, and at least part of his design uses internal treading of the tube.
Based on y'alls comments, I think I'll give up on the work-hardening or annealing/quenching approaches to get coupler nuts locked mechanically inside the tubes, since it's not a reliable or repeatable process. I'll take a look at the Swagelok butt splices; I used to do a lot of instrument tubing work with stainless and copper tubing and Swagelok fittings back in my younger days. I'm not familiar with the butt splces, though; I'll have to investigate that.
Also, the expansion fittings that I'm familiar with are the Hilti products used to anchor pipe supports, but they are single-ended. I'll check to see what exists in that area as well.
In this application, the strength of the joint is more important than electrical conductivity, since I use washers between the tube sections to lock them, and RF current flows on the outside surfaces of the tubes due to the skin effect. I'm no longer pursuing the metal-to-metal conductivity inside the tube as a design objective. Mike, your Gorilla Glue suggestion is looking more and more practical, since it swells after it is applied, and is tough! I've also tried JB Weld metallized epoxy, but it doesn't swell as much (if at all).
So thanks again to all who replied, and 73 (best wishes in ham radio lingo)
